srjami tan-niyukto 'ham-
haro harati tad-vasah
srjami—create; tat-niyuktah—engaged by Him; aham—I; harah—Lord Siva; harati—annihilates; tat-vasah—under His control; visvam—the whole universe; purusa-rupena—in the form of Lord Visnu; paripati—maintains; tri-sakti-dhrk—the controller of the three modes of material nature.
" 'Lord Brahma said, "I am engaged by the Supreme Personality of Godhead to create. Following His orders, Lord Siva dissolves everything. The Supreme Personality of Godhead, in His form of Ksirodakasayi Visnu, maintains all the affairs of material nature. Thus the supreme controller of the three modes of material nature is Lord Visnu." '
This is a quotation from Srimad-Bhagavatam (2.6.32). Lord Brahma gave this information to Devarsi Narada when he was receiving instructions from Lord Brahma to understand the Supreme Personality of Godhead, Paramatma. After describing the universal form of the Lord, Lord Brahma explained that his position and Lord Siva's position are controlled by Lord Visnu.
Link to this page: https://prabhupadabooks.com/cc/madhya/20/318